Method of preparing organic luminescent materials stabilized by heat treatment and meaterials thus obtained

ABSTRACT

The present invention relates to methods of preparing and stabilizing organic luminescent materials by means of heat treatment (annealing) in a suitable atmosphere. The treatment performed on films of such materials deposited on an inert support causes an immediate increase of the luminescence and a stabilization of the emission in ambient atmosphere for at least 500 hours.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods of preparing stabilizedluminescent organic materials, methods of stabilizing the luminescenceof organic substances, materials obtained by means of said methods anddevices (OLEDs or Organic Light Emitting Devices) and articlescomprising such materials.

PRIOR STATE OF THE ART

Over the last decade, organic compounds attracted the attention of thescientific and technological community as luminescent materials havingefficiencies comparable to those of the well-known inorganic materials,such as the semiconductors and the insulating solids used to date forthe preparation of LEDs (Light Emitting Diodes). In particular, organiccompounds have been used in the field of electroluminescence, i.e. anelectric current flow induced emission, with results so momentous thatat present several optoelectronic devices have been made with thesematerials in lieu of the usual semiconducting diodes (J. R. Sheats, H.Antoniadis, M Hueschen, W. Leonard. J. Miller, R. Moon, D. Roitman, andA. Stoking, Science, 273, 884-888 (1996); M. J. Felton, Today's Chemistat Work, 31-34, November 2001). The reason of this success lies in thatthe organic materials, besides possessing a high luminescent efficiency,a prerequisite for all luminescent materials, require elementary basictechnology for their preparation and the cost of the material isnegligible. However, aside from several advantages these materialsentail some drawbacks restricting their applicative use. Without goinginto the details of their operation it is well-known that theirluminescent efficiency decreases over time, under electrical excitationas well as owing to effects due to atmospheric moisture and oxygen (F.Papadimitrakopoulos, X. -M. Zhang, Synthetic Met. 85, 1221-1224 (1997);M. Schaer, F. Nuesch. D. Berner, W. Leo, and L. Zuppiroli, Adv. Funct.Mater. 11, 116-121 (2001)). In fact, a device unprotected fromatmosphere has an average life of 1-10 hours, whereas the same devicesuitably encapsulated has an average life of about 5000 hours. However,this life, which in some cases has been exceeded of a x2 or x3 factor,is not yet deemed sufficient to justify a generalized applicative use ofthese molecules (M. J. Felton et al supra). Moreover, it should bementioned that any encapsulation falls short of perfectly sealing thedevice over an arbitrarily long time, and moreover cannot prevent alengthy yet inescapable internal degassing process.

A method of stabilizing organic luminescent substances is disclosed inIntl. Pat. Appln. PCT/IT02/00504 to ENEA. This prior method foresees theuse of compounds of phenolic origin preventing degradation of theorganic luminescent materials and prolonging luminescence duration.

Moreover, it is known that a heat treatment (annealing) on a film ofphotoluminescent material causes a certain stabilization of theluminescent emission with regard to hydrolysis, however at the expenseof the emission efficiency (F. Papadimitrakopoulos, X. -M. Zhang, and K.A. Higginson, IEEE J. of Selected Topics in Quant. Electron. 4. 49(1998)). These prior methods already provide a remarkable improvement ofthe luminescence efficiency. Yet, there subsists the demand formaterials characterized by an even greater emission intensity and aneven longer half-life (luminous efficiency halving time) and enabling ageneralized practical application.

Hence, scope of the invention is to meet this demand.

In particular, scope of the invention is to produce luminescentmaterials having a high efficiency and an half-life longer than that ofprior art materials.

SUMMARY OF THE INVENTION

The invention is based on the unforeseen finding that the luminescenceof specific organic substances can effectively be increased andstabilized via a heat treatment (annealing) in an atmosphere havingvariable moisture contents at a predetermined temperature and controlledexperimental conditions.

In particular, the method according to the invention enables to markedlyincrease the emissive power of organic substances deposited in thin filmon a solid support. This increased efficiency is the sum of twoadvantageous and totally unexpected effects, specifically: a netimmediate increase of the luminescent emission intensity of the film,even for a film freshly-prepared and therefore of already goodluminescence, and, even more significantly, a near-constant holding ofthe emission intensity on maximum values for at least 500 hours in theabsence of any kind of protection: a result never observed before.

Hence, the main object of the invention is a method of preparing anorganic material capable of emitting luminescence comprising the stepsof: depositing a thin film of organic luminescent substance on an inertsolid support, then annealing the substance in a humidified or anhydrousatmosphere at a predetermined temperature and for suitable periods oftime, and returning to room temperature in the same atmosphere the thustreated substance in the shortest possible time, anyhow not longer than5 min. The heat treatment is conducted at a temperature ranging from 120to 180° C. in an atmosphere of air, oxygen, nitrogen or any other inertgas or mixture thereof containing different moisture levels.Advantageously, the organic substance used is a fluorescent substancelike Alq₃ or its derivatives, or equivalent substances, alone or in amixture.

A second object of the invention is a method of stabilizing theluminescence of luminescent substances, comprising the step of annealingthe substance in a humidified or anhydrous atmosphere at a predeterminedtemperature and for suitable periods of time, and returning to roomtemperature the substance thus treated in the shortest possible time,anyhow not longer than 5 min, in the same atmosphere.

Further objects of the invention are the above methods of preparing andstabilizing comprising further forms of luminescence stabilization, likethe use of phenolic substances in a mixture with the organic luminescentsubstances.

Other objects of the invention are the organic materials obtained bymeans of the disclosed method, having stabilized luminescence, filmsmade of said materials on an inert support, devices incorporating saidfilms and optionally encapsulated in moisture- and external gas-proofsystems.

Other objects of the invention will become apparent in the light of thedetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1, 4, 5, 6, 7, 12, 13, 14, 15 and 16 reproduce graphs in which thenormalized intensity of the luminescent emission (y axis) is reported asa function of time (x axis).

FIGS. 2 and 3 reproduce graphs in which the normalized intensity of theluminescent emission in arbitrary units (y axis) is shown as a functionof temperature (x axis).

FIGS. 8, 9, 10 and 11 reproduce graphs in which the intensity of theluminescent emission in arbitrary units (y axis) is shown as a functionof the emission wavelength (x axis).

FIG. 1. The figure illustrates the photoluminescence at room temperatureof two films of Alq₃, controlled for 500 hours in air (dark circles) andin a dry hood (light circles) after the evaporation process. The Alq60sample is 140 nm thick.

FIG. 2. The figure illustrates the photoluminescence at room temperatureof three films of Alq₃ heated (annealed) for 10 min in air, oxygen andnitrogen at the indicated temperature. At the start of the measuring theAlq35 and Alq41 samples were 30 and 130 nm thick, respectively.

FIG. 3. The figure illustrates the photoluminescence at room temperatureof three films of Alq₃ heated (annealed) for 10 min in air, oxygen andnitrogen humidified by water gurgling, at the indicated temperatures. Atthe start of the measuring the Alq52 samples were 110 nm thick.

FIG. 4. The figure illustrates the photoluminescence at room temperatureof one Alq₃ film measured in air for 500 hours after the heating(annealing) in a humidified atmosphere for 20 min at 150° C. Intensitypoint 1,0 corresponds to the relative value of the emission just priorto the heating. Prior to the treatment the film was 40 nm thick.

FIG. 5. The figure illustrates the photoluminescence at room temperatureof an Alq₃ film measured in air for 500 hours after the evaporationprocess. The Alq61 sample is 45 nm thick.

FIG. 6. The figure illustrates the photoluminescence at room temperatureof the Alq55 sample measured for the first 6 hours after the heating(annealing) in a humidified atmosphere for 20 min at 150° C. Point 1corresponds to the relative value of the emission just before theheating. Prior to the treatment the Alq55 sample was 40 nm thick.

FIG. 7. The figure illustrates the photoluminescence at room temperatureof an Alq₃ film measured in air for the first 6 hours after theevaporation process. The Alq61 sample is 45 nm thick.

FIG. 8. The figure illustrates emission spectra at room temperature ofthe Alq65-1 sample before and after annealing in dry oxygen at theindicated temperature. Vertical lines approximately indicate the maximaof the curves. The band at room temperature, RT, refers to the situationbefore the annealing process.

FIG. 9. The figure illustrates emission spectra at room temperature ofthe Alq65-2 sample before and after annealing in humidified oxygen atthe indicated temperature. Vertical lines approximately indicate themaxima of the curves. The band at room temperature, RT, refers to thesituation before the annealing process.

FIG. 10. The figure illustrates emission spectra at room temperature ofthe Alq65-3 sample before and after annealing in dry nitrogen at theindicated temperature. Vertical lines approximately indicate the maximaof the curves. The band at room temperature, RT, refers to the situationbefore the annealing process.

FIG. 11. The figure illustrates emission spectra at room temperature ofthe Alq65-4 sample before and after annealing in humidified nitrogen atthe indicated temperature. Vertical lines approximately indicate themaxima of the curves. The band at room temperature, RT, refers to thesituation before the annealing process.

FIG. 12. The figure illustrates the photoluminescence at roomtemperature of the Alq65-1 sample measured in air for 500 hours afterthe annealing in dry oxygen for 10 min at 180° C.

FIG. 13. The figure illustrates the photoluminescence at roomtemperature of the Alq65-2 sample measured in air for 500 hours afterthe annealing in humidified oxygen for 10 min at 155° C.

FIG. 14. The figure illustrates the photoluminescence at roomtemperature of the Alq65-3 sample measured in air for 500 hours afterthe annealing in dry nitrogen for 10 min at 180° C.

FIG. 15. The figure illustrates the photoluminescence at roomtemperature of the Alq65-4 sample measured in air for 500 hours afterthe annealing in humidified nitrogen for 10 min at 145° C.

FIG. 16. The figure illustrates the photoluminescence at roomtemperature of the Alq63-3 sample measured in air for 500 hours rightafter the evaporation process.

DETAILED DESCRIPTION

The invention, in a first embodiment thereof, consists in a method ofpreparing an organic material, deposited in form of a luminescent thinfilm, having a high emissive power stabilized over time. A secondembodiment consists in a method of stabilizing the luminescence of anorganic material previously deposited in form of thin film on an inertsupport. A further embodiment consists of a method of regenerating afilm of organic luminescent material that, due to the exposure todeactivating agents, has a luminescent emission too low for a practicalapplication.

The organic luminescent substances in accordance with the presentapplication are photoluminescent, electroluminescent or chemoluminescentsubstances. Specifically, they are metal chelates of the M(QO)n type,where M is the metal, QO is the hydroxyquinoline and n equals theoxidation state of the metal M. In the preferred embodiment of theinvention, the luminescent substance is tris-(8-hydroxyquinoline)aluminum (Alq₃), its derivatives, like thephenoxy-bis-(8-hydroxyquinoline) aluminum (Alq2-OPh),5,10,15,20-tetraphenyl-21H,23H-porphyne/Alq3 (TPP)/Alq3 complex, orfunctionally equivalent substances. Alternatively, the organicluminescent substance is replaced by a mixture of two or more of theabovementioned substances, or by a mixture comprising additionalsubstances capable of modulating the color or other aspects of theemission. Examples of such substances are tetracene, anthracene,carbazole, rubrene, TBD, PKV, DMC, α-6T, Er(TTA)₃(phen).

These mixtures can further comprise anti-oxidizing stabilizingsubstances of phenolic origin exhibiting no absorption bands on the sameemission region of the organic luminescent substances. Examples of suchphenolic substances are: phenol, vanillin, L-tyrosine, butylatedhydroxyanisole (BHA), butylated hydroxytoluene (BHT), vitamin E, propylgallate, 2,4,6-tri-t-butylphenol, hydroxytyrosol, caffeic acid. Theabovementioned substances or their mixtures are used for the preparationof thin luminescent films deposited on suitable solid supports. Thedeposition method is well known to a person skilled in the art and itconsists in a thermal evaporation/sublimation process under vacuum ofthe pure substances or of mixtures thereof in form of powder. Theevaporation process is carried out in molybdenum crucibles overtopped bythe solid substrates kept at room temperature and at a distance from thecrucible of from 10 to 30 cm, e.g. of 10, 12, 20, 28, 30 cm. The vaporsof the organic substance or mixture condense on said substrates formingluminescent material films having thickness of from 10 to 150 nm,preferably of less than 100 nm, in the optimal form from 45 to 55 nm.The material deposited in form of film and obtained with theabovementioned method has, particularly when containing no phenolicstabilizers, a high sensitivity to atmospheric agents. The luminescenceof these films, kept in air or other gas atmosphere containing normalmoisture rates, decays quickly and progressively with a half-life of afew hours, as it is shown in FIG. 1. Moreover, the decay kinetics of theluminescent emission is markedly influenced by temperature. A 10-minheat treatment in an air atmosphere containing normal moisture rates,i.e. of about the 50%, causes the complete deactivation of theluminescence already below a 250° C. temperature, as it is illustratedin FIG. 2. The Figure shows in a semi-log graph the photoluminescencepatterns for three films in air, anhydrous oxygen and anhydrousnitrogen, measured after each heat treatment at increasing temperaturesup to 250° C. While anhydrous oxygen and nitrogen do not appear to havesubstantial effects, though anhydrous oxygen attenuates luminescencemore markedly, air containing about 50% moisture completely annihilatesthe emissive capabilities of the film, whereas the same film isphysically destroyed above about 200° C. (FIG. 2).

Accordance to the invention, the luminescent films obtained as abovedescribed can be stored in an anhydrous environment for periods even ofone year, or subjected, right after their production and optionallyafter having measured their luminescent power, to a subsequent heattreatment. This treatment, called annealing, is conducted in anatmosphere of air, oxygen, nitrogen, inert gas or mixtures thereofcontaining more than 50% moisture, preferably more than 60%, 70% or 80%,and still more preferably in a moisture-saturated atmosphere. Theoptimum moisture level is attained by passage or gurgling of the air orother gas directly in a water bath. The air may be partially orcompletely replaced by pure oxygen, pure nitrogen or other gas or theirmixtures. These treatments can likewise be performed in anhydrousatmospheres.

The heat treatment is performed with any heating means enablingtemperature control, like a thermo-regulated oven provided with an inletfor the introduction of the suitable atmosphere. A few minutes ofheating, e.g. 5 to 30 min, yet preferably about 10 min, suffice tocomplete the step of annealing.

Experimental data reported in the present application unexpectedlydemonstrated that, while a heat treatment of a mere 10 min at increasingtemperature in a low moisture percent (e.g., 50%) atmosphere causes thetotal decay of the luminescence (FIG. 2), the same treatment in amoisture-rich atmosphere causes, within certain temperature limits, adramatic increase of the luminescent emission intensity followed, owingto a further temperature increase, by a fast and total decay of the same(FIGS. 3, 9 and 11). An equivalent increase of the emission intensity isattained also operating in an anhydrous atmosphere of oxygen or ofnitrogen (FIGS. 8 and 10). Therefore, for each luminescent material usedthe annealing temperature should be such as to produce said increase inthe emission intensity, without anyhow overstepping the thresholdresulting in the subsequent deactivation of the luminescence. Hence, apreliminary study as illustrated in FIG. 3 for each luminescent materialand/or atmosphere used allows to identify the temperature intervalssuitable to attain increases of the intensity of the film luminescence,which increases in the case of Alq₃ amount to a factor from 2 to morethan 4 (the maximum value of 4.13 was attained for a film annealed indry nitrogen). Experimental data demonstrated that when the luminescentmaterial be or mainly contains the Alq₃ molecule this temperature shouldbe held within the range of 150°±30° C., preferably of 140° to 170° C.,e.g. at 150° C. when operating in air. Temperatures of from 145° to 180°C. yield optimal results even operating in atmosphere of humidified aswell as anhydrous oxygen or nitrogen.

After the step of annealing, the films of fluorescent material thustreated are kept in the same humidified atmosphere for a period of timesufficing to return them to room temperature, in general of someminutes, however no more than 5. Experimental data reported in theexamples demonstrate not only that the keeping in humidified air in noway alters the entity of the emission, but also that the luminescence isstabilized for more than 500 hours at the optimum levels reached duringthe step of annealing, as illustrated by FIGS. 4, 6, 12, 13, 14 and 15.On the contrary, keeping films not heat-treated in the same experimentalconditions after their production causes an immediate deactivation downto practically useless emission levels, as illustrated by FIGS. 5, 7 and16.

What is known from preceding studies at present falls short ofconclusively explaining all the results reported in the presentapplication. For example, the luminescence decrease above a certaintemperature (FIGS. 2 and 3) may be ascribed to the action of waterreacting with Alq₃ to produce 8-Hq, 8-hydroxyquinoline, a product morevolatile than Alq₃ that evaporates violently destroying the entire film.

In order to explain the results related to the increase of emissionintensity illustrated in FIG. 3, a further series of tests was conductedusing freshly-prepared films, obtained as described in the examples.These were subjected solely to one or two heating temperatures, one ofwhich in the range in which the increases in luminescence had previouslybeen measured. Thus, there were eliminated possible positive or negativeeffects of the subsequent heating performed in the tests illustrated inFIGS. 2 and 3 and any reversibility phenomena due to a lessfreshly-prepared film.

The new results obtained not only confirmed those previously observedconcerning the photoluminescence increase, but highlighted, whollyunexpectedly, that the entity of the increase varies from a factor 2 tomore than 4 in the different cases. Moreover they showed that theluminescence band spectrally shifts toward wavelengths in the blue field(FIGS. 8, 9, 10 and 11). This shifting highlights a phase transformationof the luminescent material that can also explain the phenomenon of theincrease in the emission intensity.

In fact, it is known that the Alq₃ material can be aggregated in thedifferent crystalline phases α, β, γ, and δ, or C, A and B [M. Coelle,J. Gmeiner, W. Milius, H. Hillebrecht, and W. Brutting, “Thermal andStructural Properties of the Organic Electroluminescent MaterialTris(8-hydroxyquinoline)aluminum (Alq₃)” Proc. 11^(th) InternationalWorkshop on Inorganic and Organic Electroluminescence, EL 2002, Ghent,Belgium, Sept. 23-26, 2002, pags. 133-136], besides possessing differentspectroscopic and morphologic properties. In fact, while the α phaseemits light rather shifted in the red spectrum, the δ phase emits lightswith wavelength more shifted toward the blue with greater efficiency andmore stability to the degradation processes.

Without wishing to bound the invention to theory, in the light of theforegoing we can reasonably suppose that the annealing processtransforms part of the α-phase initially present in the film intoδ-phase, thereby explaining both the luminescence increase and theshifting to blue. The different absolute values of these variations candepend on several factors, such as the initial morphological compositionof the films determined by the evaporation conditions (duration,temperature, distance from the crucible, etc.) as well as on the type ofatmosphere in which the annealing process is conducted. The differentinitial morphological composition of the films can also explain theirdifferent behaviour towards degradation, which can be more or less rapiddepending on the composition of the phases.

In any case, though with different initial luminescence increases, allof the heated films degrade more slowly than the unheated ones, as theycomprise a greater quantity of the δ component that is stabler than theα one.

The results highlighted above are all the more surprising, in view ofthe fact that a consistent increase of the luminescence and slowing downof the degradation can be achieved already at temperatures in theneighborhood of 150° C., whereas previous experiences indicated that theδ phase was obtained by sublimation and annealing at about 385° C., atemperature far higher and impracticable for films, let alone forcompleted devices.

The stabilized luminescent material films obtained as described heretoare used for the preparation of devices, called OLEDs, luminescent whensuitably excited, e.g. by electric current flow.

The scheme of a typical OLED device is disclosed, e.g., in the precedingIntl. Appln. PCT/IT02/00504 or in literature. Such a scheme foresees amultilayer structure comprising the support, a layer containing theanode, a layer that easily transports holes (also indicated as HTL), aluminescent layer (also indicated as LL), a layer that easily transportselectrons (also indicated as ETL) and a layer containing the cathode.Any organization change in the structure of the device is encompassed bythe present invention, insofar as the luminescent layer is in accordancetherewith.

Optionally, the luminescent devices can be encapsulated in outsideatmosphere-proof systems capable of sealing the luminescent material offfrom contact with atmospheric agents.

The invention is disclosed hereinafter by way of non-limiting examplesthereof.

EXAMPLES 1 TO 7

Different samples of luminescent films were produced byevaporation/sublimation under vacuum of the Alq3 substance andsubsequently treated in accordance with the present invention. In thiscase, the only devices processed consisted of a single layer ofluminescent material deposited on a support, or at most of two layersthe second one of which having a protective function. The samples madefor this study, freshly-prepared or long time-prepared, are reported inTable 1. TABLE 1 Sample keeping Thick- in the Annealing ness Evaporationmean- date and EXAMPLES (nm) date time atmosphere Example 1: Alq35  30Jan. 10, 2001 drier Jan. 04, 2002 O₂ Example 2: Alq41 130 Feb. 12, 2001drier Dec. 22, 2001 air — — — — Dec. 24, 2001 N₂ Example 3; Alq52 110Jan. 10, 2002 drier Dec. 12, 2002 Humidified air — — — — Jan. 15, 2002Humidified N₂ — — — — Jan. 17, 2002 Humidified O₂ Example 4: Alq55  40Jan. 19, 2002 drier Jan. 29, 2002 Humidified air Example 5: Alq60 140Jan. 05, 2002 none Apr. 05, 2002 Example 6: Alq61  45 Apr. 10, 2002 noneApr. 10, 2002

The Alq₃ substance used in the examples is of commercial origin(Aldrich) as well as freshly-synthesized by means of known methods.

For the preparation of the films there were used microscope glass slidespretreated in a degreasing ultrasonic bath and then with solutions ofsurfactants, acids, potassium bichromate and alcohols in subsequentsteps alternate to rinsings in doubly distilled water and thereafterdried in a flux of nitrogen gas. The deposition of the fluorescentsubstance in a film was performed by evaporation under vacuum (1.6×10⁻⁶torr) operating in molybdenum crucibles at a temperature sufficing tocause evaporation/sublimation of the Alq₃ powders, for times of fromabout 10 minutes to some hours, whereas the support was held above thecrucible at a distance of from 10 cm to 30 cm and at room temperature.

The thickness of the films, reported in Table 1, can suitably becontrolled both during the growth, by means of a Varian model n.985-7019 Thickness Monitor, and after the growth, by using a TencorAlphastep profilometer. Other measuring instruments are available on themarket and known to a person skilled in the art. The films thus preparedwere dismounted from the evaporation apparatus and stored in a drier atroom temperature and in an anhydrous atmosphere prior to being processedin accordance with the invention. The subsequent step of annealing wasconducted subjecting the film to a heat treatment for about 10 min, atabout 150° C., in a moisture-saturated atmosphere obtained inlettingair, pre-passed through a water bath, into the heating apparatus. Afterthis heating step the films were returned to room temperature and keptin the same saturated atmosphere for more than 5 min.

The measurements of the optical adsorption of films treated or untreatedin accordance with the invention were performed by using a Perkin-Elmer119 spectrophotometer. The luminescent emission was measured by using aJobin-Yvon Fluorolog-3 spectrofluorometer with a front-facing detectinggeometry where both the excitation at 395 nm and the luminescence insistfrom the same side of the thin film with an angle of about 20° betweenthe geometric axes. All measurements were performed in open atmospherewithout any permanent protection of the thin film and at roomtemperature, the performing of each measurement requiring about 5 min.With the exception of the measurement times and of the time required todismount the just-prepared film from the evaporation apparatus, about 5min, pending further treatment, all films were kept at room temperaturein a dry box. This was meant to avoid any forming of non-luminescentcomplexes requiring some hours of exposure to the moisture-containingatmosphere in normal laboratory conditions.

The effects of atmosphere on the luminescence of an unprotected film,Alq60, are reported in FIG. 1, both in the case of an atmosphere in alaboratory environment (dark circles) and in that of the same atmospherein a dry hood (light circles).

In a second experiment, the Alq41 and Alq35 films were heated for 10 minin an atmosphere of anhydrous oxygen, anhydrous nitrogen and aircontaining ≦50% moisture. The results are reported in FIG. 2. Theheating to >150° C. in a low-moisture atmosphere causes the fast andcomplete deactivation of the luminescent emission.

Other three samples of the Alq52 film were subjected to heat treatmentin different moisture saturated atmospheres (about 100% water),precisely in air, oxygen and nitrogen. The results are reported in FIG.3. The effect of humidity is dramatic, as all three films are completelydestroyed at different temperatures, all below about 200° C. Yet,contrarily to what is reported in FIG. 2, prior to the irreversible dropof the luminescence, a substantial increase of the same is observed; infact, it should be noted that the scale of the intensities islogarithmic.

In a further experiment, the Alq55 film was directly heated for 10 minat 150° C. in a humidified atmosphere with the result of increasing itsemission intensity of the 70%, as expected in view of the measurementsreported in FIG. 3. After this treatment the sample was returned to roomtemperature in humidified air in the shortest viable time, and after thefirst luminescence measurement kept at room temperature in humidifiedair for 20 min undergoing no emission decrease, then left in openatmosphere for 500 hours, i.e. approximately three weeks. FIG. 4 showsthe pattern of the photoluminescence of this sample, having taken theinitial value prior to the annealing process as value 1.

By way of comparison, a second sample, the Alq61 film, not subjected toany heat process after the initial evaporation, was kept in openatmosphere like the preceding Alq55 film for the same time. FIG. 5reports its photoluminescence pattern over time. With respect to thepreceding sample subjected to annealing in humidified air, a dramaticdrop in intensity is apparent. Yet, even more surprising is the patternof the luminescence of the two samples compared in the initial hours ofthe experiment. These patterns are highlighted in FIGS. 6 and 7 thatrelate to the initial 6 hours of the Alq55 and Alq61 samples. The Alq55sample, after a luminescent intensity increase of about the 70%,exhibits no apparent drop in the experimental time, contrarily to theAlq61 sample, which immediately exhibits a significant decrease inemission. To this interesting result it should be added that the first20 min of the Alq55 sample after the annealing process elapsed in ahumidified atmosphere undergoing no decay, whereas the same processwould have been disastrous for an unheated film.

EXAMPLES 7 TO 10

In a second series of experiments, new films were produced in accordancewith the method described in the preceding examples, but adopting theexperimental conditions reported herebelow. The films were generated byevaporation of Alq₃ powder (Aldrich), and for the different evaporationstheir thickness ranges from 45 to 55 nm. For each evaporation fouridentical films designated AlqMN-O are produced, in which MN is thesequence number of evaporation, whereas O is 1, 2, 3, 4 indicates one ofthe four films. All samples were used right after their depositing, orat latest within a few days, having been stored under vacuum or in adryer. The annealing measurements were performed in different ambientconditions and at optimal temperatures predetermined for each film, asindicated in Table 2, but always for a period of 10 minutes after havingattained the thermal and atmospherical balance typical of the individualmeasurement. The adsorption and the emission with excitation at 395 nmwere measured for each sample. Then the optical measurements werecarried on in order to study the decay of the various films kept in roomatmosphere. The samples generated for this study are reported in Table2, and the results illustrated by FIGS. 8 to 16. TABLE 2 Example 7Alq65-1 Heating In dry oxygen at 180° C. Example 8 Alq65-2 Heating Inhumidified at 155° C. oxygen Example 9 Alq65-3 Heating In dry at 180° C.nitrogen Example 10 Alq65-4 Heating In humidified at 145° C. nitrogenComparative Alq63-3 No Heating Example 11

FIG. 8 shows the emission bands of the Alq65-1 sample before and afterthe annealing process in dry oxygen. Two quite important details areimmediately evident. After the annealing process, the intensity of theband peak increases of a factor of about 3.7 and the wavelength of themaximum shifts into blue of about 12 nm. FIGS. 9, 10 and 11, referringto humidified oxygen, dry nitrogen and humidified nitrogen,respectively, show the same general variations, though with numericalvalues different thereamong.

FIG. 12 shows the pattern over time in air of the maximum intensity ofthe emission band of sample Alq65-1 after the annealing process in dryoxygen, monitored for 500 hours. FIGS. 13, 14 and 15 show the samepatterns for the other samples, in that order. For a significantcomparison, FIG. 16 shows the same pattern for the Alq63-3 sample thathas not been subjected to any annealing process, and therefore that canbe taken as reference.

The above described results, and many comparable other results obtainedin the same test cycle, though not reported here confirm that:

the annealing process at about 150° C. in any type of atmosphereincreases of a factor 2 to 4 the intensity of the film photoluminescence(the maximum value of 4.13 was attained for another film annealed in drynitrogen),

after the annealing process the barycenter of the band shiftsconsistently toward the blue field of the spectrum.

the films subjected to the annealing process degrade over time moreslowly than the freshly-evaporated films, as it is evident from acomparison of the data reported in FIGS. 12, 13, 14 and 15 with thoserelated to the comparative product of FIG. 16,

there are differences among samples showing the existence of processesdue to the type of atmosphere used in the annealing.

1. Method of preparing an organic luminescent material comprising thesteps of depositing a thin film of organic luminescent substance on asolid inert support, then heat-treating (annealing) the depositedsubstance at a predetermined temperature in a humidified or anhydrousatmosphere and, finally, returning to room temperature keeping thesubstance in the same atmosphere.
 2. Method according to claim 1,wherein the humidified atmosphere is an atmosphere of oxygen, nitrogen,air, inert gas or mixture thereof, containing more than 50% moisture, orof anhydrous oxygen, nitrogen, or inert gas or mixtures thereof. 3.Method according to claim 2, wherein the annealing is performed at apredetermined temperature ranging from 120° C. to 180° C. for a periodof time of 5 to 30 minutes, followed by returning to room temperature ina time not longer than 5 minutes.
 4. Method according to claim 3,wherein the predetermined temperature of annealing is the temperatureproducing the maximum increase of the emission intensity of the film. 5.Method according to claim 4, wherein the organic luminescent substanceis a photoluminescent or electroluminescent substance selected from thegroup consisting of tris-(-8-hydroxyquinoline) aluminum (Alq3),phenoxy-bis-(8-hydroxyquinoline) aluminum (Alq2-OPh),5,10,15,20-tetraphenyl-21H, 23H-porphine/Alq3 (TPP)/Alq3 complex,functionally equivalent substances and mixtures thereof.
 6. Methodaccording to claim 5, wherein the organic luminescent substanceoptionally comprises one or more substances selected from the groupconsisting of tetracene, anthracene, carbazole, rubrene, TBD, PKV, DMC,α-6T and Er(TTA)3(phen) and/or one or more phenolic compounds capable ofstabilizing luminescence selected from the group consisting of phenol,vanillin, L-tyrosine, butylated hydroxyanisole (BHA), butylatedhydroxytoluene (BHT), vitamin E, propyl gallate,2,4,6-tri-t-butylphenol, hydroxytyrosole, and caffeic acid.
 7. Methodaccording to claim 1, wherein the film of organic luminescent substanceis generated by evaporating/sublimating under vacuum the luminescentsubstance and depositing the vapors on an inert flat support of glass,crystal, plastics material or any other substance compatible withadsorption, emission and detection of light.
 8. Method according toclaim 1, wherein the organic luminescent substance comprises Alq3, theannealing is performed at about 150° C. for about 10 minutes, thehumidified atmosphere is water-saturated air and after annealing thefilm is returned to room temperature in the same atmosphere in a timenot longer than 5 minutes.
 9. Method of stabilizing the luminescence ofan organic photoluminescent or electroluminescent substance comprisingannealing the substance at a predetermined temperature in a humidifiedor anhydrous atmosphere and then returning to room temperature keepingthe substance in the same atmosphere.
 10. Method according to claim 9,wherein the humidified atmosphere is an atmosphere of oxygen, nitrogen,air, inert gas or mixture thereof containing more than 50% moisture, orthe anhydrous atmosphere is an atmosphere of oxygen, nitrogen, inert gasor mixture thereof.
 11. Method according to claim 10, wherein theannealing is performed at a predetermined temperature ranging from 120°to 180° C. for a period of time of 5 to 30 minutes, followed byreturning to room temperature in a time not longer than 5 minutes. 12.Method according to claim 11, wherein the predetermined temperature ofannealing is the temperature producing the maximum increase of theemission intensity of the film.
 13. Method according to claim 9, whereinthe organic luminescent substance is a photoluminescent orelectroluminescent substance selected from the group consisting oftris-(-8-hydroxyquinoline) aluminum (Alq3),phenoxy-bis-(-8-hydroxyquinoline) aluminum (Alq2-OPh),5,10,15,20-tetraphenyl-21H, 23H-porphine/Alq3 (TPP)/Alq3 complex,functionally equivalent substances and mixtures thereof.
 14. Methodaccording to claim 13, wherein the organic luminescent substanceoptionally comprises one or more substances selected from the groupconsisting of tetracene, anthracene, carbazole, rubrene, TBD, PKV, DMC,α-6T and Er(TTA)3(phen) and/or one or more phenolic compounds capable ofstabilizing luminescence selected from the group consisting of phenol,vanillin, L-tyrosine, BHA, butylated hydroxytoluene (BHT), vitamin E,propyl gallate, 2,4,6-tri-t-butylphenol, hydroxytyrosol, and caffeicacid.
 15. Method according to claim 14, wherein the organic luminescentsubstance subjected to annealing is in form of film deposited on aninert flat support of glass, crystal, plastics material or any othersubstance compatible with adsorption, emission and detection of light.16. Method according to claim 9, wherein the organic luminescentsubstance comprises Alq3, the annealing is performed at about 150° C.for about 10 minutes, the humidified atmosphere is water-saturated airand after annealing the film is returned to room temperature in the sameatmosphere in a time not longer than 5 minutes.
 17. Organic luminescentmaterial having a stabilized luminescence, obtainable by the method ofpreparing according to claim 8 and being in a crystalline phasedifferent from that of the original luminescent substance.
 18. Organicluminescent material having luminescence stabilized with the method ofstabilizing according to claim to 16 and being in a crystalline phasedifferent from that of the original luminescent substance.
 19. Film oforganic luminescent material according to claim 17, deposited on aninert support.
 20. Film of organic luminescent material according toclaim 18, deposited on an inert support.
 21. Luminescent devicecomprising the film according to claim
 19. 22. Luminescent devicecomprising the film according to claim
 20. 23. Sealed atmospheric agentproof system comprising the device according to claim 21 in an inertatmosphere.
 24. Sealed atmospheric agent proof system comprising thedevice according to claim 22 in an inert atmosphere.
 25. Use of deviceaccording to claim 21 for the preparation of electrooptic articles. 26.Use of system according to claim 23 for the preparation of electroopticarticles.